A single LED die intended for solid state illumination is generally made from one of two semiconductor materials: InAlGaP (red, orange and amber) and InGaN (green and blue). Similar to standard PN junction diodes, LEDs conduct current when they are forward biased. Two design aspects of LEDs are (1) that they are driven by current, and (2) the forward voltage (VF) is low and DC; typical VF ranges from 2V to 3V for InAlGaP LEDs and from 3V to 4V for InGaN LEDs. The luminous flux of an LED is proportional to the forward current.
Since LEDs are current driven devices, the preferred driving method is with a constant current source; this reduces changes in current due to variations in forward voltage across the semiconductor junction of the LED. In order to regulate current, the input power supply regulates the voltage across a current-sense resistor in series with the LED(s). Integrated circuits (ICs) that perform the driving function are known as LED drivers. The driver IC comprises a constant-current source (e.g. a switching regulator) and circuitry to regulate the DC voltage across the current-sense resistor. The switching regulator, also called the half-bridge stage, is controlled by circuitry (comprising analogue blocks such as a comparator, reference level generator etc., and also digital logic) which monitors the voltage on the current sense resistor. This system as a whole works as a constant current source. The half-bridge may or may not be fully or partially in the IC, but the control circuitry is embedded. The current-sense resistor is not part of the LED driver and an application designer chooses the value of the current sense resistor to be suitable for the particular application, LED(s) and driver.
The switching regulator in the LED driver has one or more power transistor which in use often has the greatest heat losses on the IC, although heat is lost from other circuit components. In order to protect the power transistor(s) and other circuit components on the IC from damage at high temperatures, the LED driver IC comprises an internal temperature sensing circuit which, ideally, is close to the power transistor(s) but not necessarily so. In use, the temperature sensing circuit provides an output signal indicative of the internal temperature in the IC which is monitored by the control logic circuit. If the internal temperature of the IC reaches a predetermined threshold (e.g. 130° C.), the control logic shuts down the switching regulator to inhibit further rises in internal temperature that might otherwise damage the power transistor(s) and other IC components. When the temperature falls below the threshold temperature, the control logic activates the switching regulator again. If the LED driver is used in environments with high ambient temperature, an on-off cycle of the LED(s) can result, which is highly undesirable from a visual perspective.
To compound this problem, LED drivers are intended for use in a diverse range of environments. For example, the LED driver can be used in freezer lighting, and in exterior and interior architectural applications amongst others. Accordingly the ambient temperature around the IC during operation may vary considerably, for example in external applications between ±20° C. to 30° C. or more depending on the location, season and time of day. The variation in ambient temperature affects the internal temperature of the IC. When in operation, heat is dissipated by the LED driver and LED(s) which increases the internal temperature of the IC above ambient, and may often approach the maximum junction temperature for the power transistors, and/or a temperature that would damage other circuit components.
The heat losses in the LED driver IC may be divided in two categories: DC conduction losses and transient losses. The conduction losses are directly proportional to the RMS value of the output current and also proportional to the duty cycle (if only one internal power transistor is used). Transient losses comprise capacitive losses and the switching loss (due to non-zero voltage switching), both of which are proportional to switching frequency. How these losses affect the internal temperature of the IC (i.e. inside the package) is non-trivial, but does depend to some extent on the IC mounting substrate and any heat sinks amongst other things.
Therefore the problem facing the LED driver designer is to match the LED driver and its external circuit components to the particular application, having regard to the expected ambient temperature variation at point of use. To that end, the designer tries to limit heat dissipation in the IC as far as possible. One way that the designer can limit heat dissipation is to adjust the components used in the LED driver to control the output power by adjusting the switching frequency. In particular, an inductor is required for the switch-mode power supply to transfer input voltage to output voltage without wasting power. By changing the value of this inductor, the rate of change of current across the inductor changes, and the application designer can increase or decrease the switching frequency.
For example, a particular LED driver might be expected to operate at ambient temperature usually of 45° C., but occasionally that temperature can rise up to 75° C. The application designer has to ensure the IC temperature never goes above 125° C. if temperature shut down is to be avoided. Therefore a 50° C. rise in temp in IC is permissible, assuming highest ambient temperature. If the thermal resistance of package is 50K/W the LED driver must be designed so that switching frequency and average current heat losses do not exceed 1 W. This can be achieved by using a larger inductor, but this also limits output light from the LED(S). Furthermore, since the ambient temperature is mostly 45° C., the LED driver operates at significantly less than maximum performance. An alternative would be to design for a 1.6 W heat dissipation, use smaller inductor, permitting the same average current and thereby output light, but at a lower switching frequency. However, this is with the risk that at higher ambient temperatures the temperature cut-off problem may occur.
In many cases the designer chooses to design for the worst case ambient temperature (i.e. the highest expected value) to reduce the chance that the driver and LED(s) will suffer the aforementioned on-off cycling problem. Therefore a conservative inductance value is used that maintains a lower average current than could be used for most of the time, affecting the performance of the LED(s). Furthermore extra heat sinks may be added as a precaution, increasing cost.
If there is any mismatch between the expected ambient temperature variation and reality the LED driver and LED may nonetheless still suffer from the on-off cycling problem. From the user's perspective this visual effect is highly undesirable.
US 2005/014315 discloses a current control device for driving LEDs that employs switch mode hysteretic control to de-rate the current with increasing temperature. The temperature external to the LED driver IC is indicated with a resistor in order to switch on and off the de-rating. Another external resistor sets the shutdown temperature for the LED driver. This arrangement monitors the temperature of the LEDs, but will not prevent the LEDs being switched on and off by an LED driver in over temperature conditions inside the IC.